The present invention relates to a process for producing a silver halide emulsion and, in particular, to a process that is capable of mass production of silver halide emulsions having consistently good photographic characteristics and properties.
Photographic silver halide emulsions are formed by mixing halide solutions with a silver nitrate solution in the presence of hydrophilic colloids such as gelatin so that a slightly soluble silver halide formed from halide and silver ions is dispersed in the hydrophilic colloid.
Silver halide emulsions are prepared by a process that typically consists of the following steps in sequence: a physical ripening step in which fine silver halide nuclei are formed and permitted to grow into larger grains; a desalting step which includes the washing out of salts, excess halide ions and other by-products from the silver halide formed in the physical ripening step; a chemical ripening step for increasing the sensitivity of silver halide by, for example, sensitization with a reduction sensitizer, a noble salt sensitizer, a sulfur sensitizer, etc. or spectral sensitization performed in the presence of a sensitizing dye; a conditioning step which includes the addition of a variety of chemicals with a view to improving coating, film and other properties; and a coating step in which the prepared emulsion is coated on a support.
In order to perform these steps on an industrial scale (e.g., 100 liters or more), a variety of apparatus and parts are necessary and they include vessels such as a reactor, agitators, piping, nozzles, transport containers, storage containers, and metering apparatus. All of these apparatus and parts have large dimensions and require large amounts of materials for their construction. In addition, the choice of the material to be used must be based on consideration of its suitability for a specific step, that is, a material having good heat transfer is selected if efficient heat transfer is important, a highly corrosion-resistant material is used in a corrosive environment, or a material having good machinability is used to fabricate a complex structure. Most importantly, none of the materials selected should cause adverse effects on the photographic characteristics (e.g., sensitivity and fog) of the silver halide emulsion to be prepared.
However, no comprehensive and detailed studies have been conducted in order to identify materials that can safely be used to construct all the equipment for emulsion preparation. Unexamined Published Japanese Patent Application No. 154437/1984 discloses a technique for preventing variation in the photographic performance of the silver halide grains being produced in the physical ripening step by using a specific material in that part of the reactor which contacts silver ions, and the material is selected from among those which are electrochemically inert to silver ions, such as Teflon, ceramics, silicon and glass lining. Japanese Patent Application No. 225462/1984 discloses the use of metallic or alloyed zirconium as a technique for preventing variation in the performance of silver halide emulsion. The methods and apparatus shown in these patents are effective in certain applications but before they can be applied to all the equipment for preparation of silver halide emulsions, many problems remain to be solved in terms of construction cost, heat transfer, temperature control, etc.
Corrosion-resistant stainless steels such as SUS 304 and SUS 316 are known to be usable as the constituent materials of commercial equipment for the preparation of silver halide emulsions. These stainless steels are based on iron and contain chromium, nickel and other alloying elements such as molybdenum. Techniques for improving the corrosion resistance of steels by addition of elements such as chromium, nickel and molybdenum are disclosed in various references such as "Handbook of Corrosion Inhibiting Technology", Kagaku-kogyo-sha, 1972, "Stainless Steel Handbook", Nikkan Kogyo shinbunsha, 1976, and "Introduction to Metal Corrosion and Its Inhibition", Kagaku Dojin, 1973. According to these references, chromium, nickel and molybdenum are able to rapidly passivate steels in which they are added and to retard subsequent dissolution of the steel components.
As mentioned previously, SUS 304 and SUS 316 are generally known to be corrosion-resistant stainless steels but if they are placed in the environment in which silver halide emulsions are produced, their components, such as iron, chromium, nickel and molybdenum, will dissolve into the emulsion in small amounts. Although the dissolution of these components is very small, the ions of heavy metals, i.e, chromium, nickel and molybdenum, cause adverse effects on the silver halide emulsion such as reduced sensitivity and increased fog even if they are only present in amounts at the ppm to ppb level.